The present invention relates in general to dynamic noise reduction of an audio signal, and more specifically to an adaptive audio filter having a continuously variable sliding bandwidth to control the energy leaving the audio filter as a fixed percentage of the energy entering the audio filter, wherein the adaptation is disabled under low modulation conditions.
Audio systems, such as radio receivers, have used dynamic noise reduction in an attempt to control background noise when low level audio signals are present or when a radio receiver receives a weak RF signal. In some systems, high frequency content of the demodulated signal is compared with a fixed threshold to detect noise, and a lowpass filter is engaged when high frequency content is above the threshold. Such systems are not effective on all types of broadcast signals and often result in audible "breathing" effects due to the switching in and out of the lowpass filter.
U.S. Pat. No. 3,889,108 issued to Cantrell discloses an adaptive lowpass filter for reducing noise in an audio signal. However, in controlling the adaptive filter, Cantrell divides an expected noise power by an RMS error. Thus, the predetermined known or estimated noise power of the noise signal that will be received is required. In practice, this value may not be subject to accurate estimation. Furthermore, the requirement to perform a division is cumbersome and expensive to implement in digital signal processing (DSP) hardware, as opposed to implementations using only multiply functions. Furthermore, Cantrell compares the difference of instantaneous signals, and thereby inadvertently includes phase difference information in the comparison which is not relevant for determining a desired bandwidth of the filter. The phase difference results in larger error signals which reduces the filter bandwidth farther than desired.